This invention relates to an apparatus for filtering high temperature gases, such as high temperature high pressure gases containing solid contaminants entrained therein and being discharged from a circulating fluidized bed reactor. The filtration apparatus typically comprises porous monolithic ceramic filter elements or the like. The present invention also relates to an apparatus for efficiently removing accumulated solids from the filtering elements in a safe manner.
It is known in the prior art to use ceramic filters, in order to remove particulates from hot gas streams. It is e.g. known to use candle type ceramic filters (as shown in U.S. Pat. No. 4,869,207) supported by a tube sheet for cleaning hot gases. The size of the filter housing is however presently limited, the practical limit for the diameter of a pressure vessel with candle type filter being about 2-5 m.
It has also been suggested to utilize monolithic ceramic filters, having e.g. a plurality of passageways extending longitudinally from inlet end to outlet end, but being plugged to prevent direct passage of the feed stock through the passageways from the inlet face to the outlet face. The cleaning capacity of clusters of such monolithic ceramic filters is much higher than of conventional candle type or tubular ceramic filters. The monolithic ceramic filters thereby being less space consuming than conventional tubular or candle type ceramic filters. The mounting of these elements in filtration vessels in high temperature surroundings and possible temperature variations has however led to very complicated constructions.
It is also known that it is essential to have the filters cleaned, e.g. after certain pre-determined operating periods, in order to be able to maintain the desired pressure drop. Commonly used methods of cleaning the filter employ a reverse directional pulse of gas for flushing the filter. This kind of method is disclosed e.g. in U.S. Pat. No. 5,284,498 showing in a filtration vessel a single integral inner shell provided with ceramic monolithic filter elements for passing clean gas from the inner dirty gas side of the inner shell to the outer annular clean gas side of the shell. A back pulsing nozzle for back pulsing cleaning gas is provided in front of each monolithic filter element.
It is an object of the present invention to provide an improved apparatus for filtering high temperature gases. It is especially an object of the present invention to provide a new way of arranging porous monolithic ceramic filters in a filtration vessel.
According to one embodiment of the present invention, an apparatus for filtering high temperature gases both from pressurized (i.e. superatmospheric pressure) systems and atmospheric systems, is provided. The filtration apparatus comprises following elements:
a hollow vessel having an inlet for dirty gas containing solid contaminants to be filtered therefrom, an outlet for clean gas and an outlet for particles filtered from the gas. shaped (i.e. torodial hollow clean gas chamber disposed in a dirty gas volume in the hollow vessel, the clean gas chamber having a generally impervious peripheral wall preventing dirty gas from flowing from the dirty gas volume through said wall into the clean gas chamber,
at least one porous ceramic filter element disposed in an opening in the peripheral wall of the clean gas chamber, allowing filtered clean gas to flow from the dirty gas volume through the filter element into said clean gas chamber, and
means for discharging clean gas from the clean gas chamber to the exterior of the hollow vessel.
The apparatus according to the present invention may advantageously utilize monolithic ceramic filter elements, such as CeraMem.TM. filters as shown in U.S. Pat. No. 5,114,581 or cross flow filters as shown in U.S. Pat. No. 5,078,760, the disclosures of which are incorporated by reference herein. CeraMem.TM. filter elements are typically cylindrical having several parallel longitudinal passageways therethrough, the passageways being plugged to prevent dirty gas from flowing directly through a passageway connected with the dirty gas side and forcing clean gas to flow through the porous ceramic material into an adjacent passageway connected with the clean gas side. Cross flow ceramic filter elements being e.g. formed of several ceramic ribbed sheets forming crosswise clean and dirty gas channels. In general, the filter elements may be constructed and mounted as in copending application Ser. No. 08/246,221 filed May 18, 1994, the disclosure of which is hereby incorporated by reference herein.
According to a preferred embodiment of the present invention, there is a generally upright hollow pressure vessel having a top, side walls and a bottom. In the vessel two or more generally torodial or doughnut shaped clean gas chambers are disposed concentrically one on top of the other spaced vertically from each other. An inlet for dirty volume is disposed in the top of the vessel generally concentrically with the doughnut shaped clean gas chambers.
Each clean gas chamber is preferably made of an elongated hollow open ended tubular element bent into a ring shaped form. The tubular element preferably has a circular cross section. The tubular element may alternatively have a semicircular, square or polygonal cross section if desired. Both ends of the tubular element are connected to each other in a gas tight joint, thereby forming a torodial chamber. The two or more doughnut shaped clean gas chambers disposed on top of each other in the vessel preferably have the same diameter and cross sectional dimensions, but may if desired have different diameters and/or dimensions.
Polygonal (e.g. square) or semicircular cross section clean gas chambers may be used instead of circular cross section clean gas chambers. In polygonal or square cross section and plan view clean gas chambers the peripheral walls of the clean gas chambers may be made of straight plate material, which in some cases is an advantage.
According to a preferred embodiment of the present invention the monolithic ceramic elements are mounted in openings provided in the peripheral wall of each clean gas chamber. The filter elements may be mounted in a row or in some other configuration one after the other on the inner, outer, top or bottom peripheral wall or walls of a ring shaped clean gas chamber. The filter elements may if desired be mounted simultaneously at different sides of the clean gas chamber. The filter elements may i.e. be arranged in zig-zag form, in turns on opposite sides of the clean gas chamber, the filters overlapping each other therein. In clean gas chambers having a semicircular peripheral cross section, i.e. having one straight side and one generally semicircular side, the ceramic filter elements are easily connected to openings on the straight side. The clean gas chambers may have partition walls therein.
The clean gas chamber may have a rather small cross sectional diameter, as long as it is possible to insert the ceramic filter elements into openings made on the peripheral walls thereof.
The porous ceramic filter elements, being e.g. cylindrical in form and having one inlet (dirty) end and one outlet (clean) end, may in some cleaning processes protrude with their clean and relatively deep into the tubular elements, such that the ceramic element may cover &gt;1/2, or even more of the cross sectional inner area of the clean gas chamber. The clean gas chambers need enough free inner space for clean gas to be able to pass the filter elements therein and flow freely into a clean gas discharge conduit. A very compact filter module, clean gas chamber with filter elements, is provided if the filter elements are arranged on the inner peripheral wall of the clean gas chamber.
Also, other advantages are achieved, especially when cleaning the ceramic filters by back pulsing, when the filter elements are disposed with their clean ends protruding rather deep, e.g. &gt;1/2 of them, into the tubular elements. Cleaning of monolithic ceramic filter elements in an apparatus according to the present invention may be made in a conventional manner e.g. as has been suggested for different candle type filters, by injecting clean gas or air into the clean gas chamber. The high pressure cleaning pulse thereby compresses the portions of the ceramic filter elements being inside the clean gas chamber from all sides, preventing mechanical breaking of the filter elements.
According to a preferred embodiment of the present invention a clean gas outlet conduit is connected to each clean gas chamber, the clean gas outlet conduit being arranged to lead clean gas from the chamber to the exterior of said hollow vessel.
According to another preferred embodiment of the present invention a connecting duct is arranged between different clean gas chambers, not all having outlet conduits connected to them, for allowing clean gas to flow from one chamber to another and ultimately to one or a few clean gas chambers having a clean gas outlet conduit connected to them. Clean gas chambers in the upper part of the hollow vessel preferably have outlet conduits leading through the top of said hollow vessel, whereas outlet conduits in the lower part lead through the bottom of the vessel.
A vertical outlet conduit from a clean gas chamber passing other clean gas chambers may be arranged to pass these clean gas chambers mainly in the middle part of the vessel i.e. in the central opening of those doughnut shaped clean gas chambers.
Several clean gas outlet conduits may be connected to each clean gas chamber, for discharging clean gas and simultaneously supporting said clean gas chambers within the filtration vessel. Alternatively each clean gas chamber may be connected by supporting bars to the filtration vessel construction.
Separate means for introducing a pulse of cleaning gas into the clean gas chambers, for back pulsing filter elements disposed in openings therein and dislodging particles which have collected on said filter elements, may be provided in each clean gas chamber. A clean gas chamber may if desired be divided into sections, such that different sections are back pulsed separately, at the same moment or at different moments. Thereby the clean gas chamber may be divided into sections by partition walls therein.
The back pulsing of the filter elements is performed so that particles fall down into the bottom of said vessel and are discharged through an outlet therein. The means for introducing a back pulse may alternatively by connected to the clean gas outlet conduits connected to the clean gas chambers, especially if a separate clean gas outlet conduit is provided in each clean gas chamber.
The present new filtration apparatus is especially suitable for cleaning dirty gases in a system comprising a pressurized fluidized bed reactor discharging gases with entrained particles and at a temperature of between about 500.degree.-1200.degree. C., and at a pressure of between about 2-40 bar.
Dirty gas, such as process gas from a combustor or gasifier, introduced centrally into the vessel through an inlet in the top thereof is easily distributed from the middle of the vessel evenly to all filter elements disposed in doughnut shaped chambers. In the vessel the dirty gas flows downwardly and simultaneously radially outwardly.
Downward flow of gas is also advantageous when considering back pulsing of the filters. Solid particulates dislodged by back pulsing is guided downward with the dirty gas toward the solid particulate outlet.
If desired it is also in a filtration vessel according to the present invention easy to guide the incoming flow of dirty gas to the periphery immediately in the top of the vessel and to guide the flow from the upper part of the vessel evenly downward adjacent the side walls of the vessel on the outside periphery of the doughnut shaped clean gas chambers. The dirty gas then flows downward along the vessel and simultaneously radially inwardly toward the filter elements. If desired the inlet of dirty gas may be arranged in a side wall of the filtration vessel.
One of the main advantages provided by a filtration apparatus according to the present invention is a symmetrical heat expansion of the doughnut shaped clean gas chambers, which minimizes many problems normally arising in filtration vessels due to changes in temperature.
One further advantage achieved with the present invention is a simple and minimal space consuming construction of a doughnut or ring shaped clean gas chamber in a circular pressure vessel.